You might want to consider some sort of "trial membership" period after the change over. This would allow newly registered fools to cruise the message boards for a set period (two weeks, one month) but not post until they suscribe. By prohibiting posting, you will keep out the spammers and the Sybils (multiple identity posters). Theoretically, someone could cheat by continiously signing up for trial periods, but you can cut that down by requiring a valid and unique e-mail address to sign up for the trial period. Furthermore, how many people will really go to all that effort just to save $30? Not enough to worry about.

You also might award "scholarships" to confirmed students, military members, etc.

The point of these suggestions is to minimize the loss of new blood that a subscriber fee might cause.